It is well-known that rotating waves of electrical activity are a factor in potentially dangerous cardiac arrhythmias such as ventricular tachycardia and ventricular fibrillations (VT/VF). The rotating waves, or reentries, that are responsible for ventricular tachycardia events are classified into two categories: 1) functional reentries, which involve freely rotating waves; and 2) anatomical reentries, where a wave rotates around an obstacle such as a blood vessel or piece of ischemic tissue. The latter are referred to as being ‘pinned’ by the obstacle. Traditional defibrillation is not a preferred way of dealing with such rotating waves because defibrillation resets electrical activity everywhere in the heart and uses high voltage shocks, which have undesirable side effects.
One common method of attempting to terminate these rotating waves or reentries is anti-tachycardia pacing (ATP). ATP has a high rate of success in dealing with functional reentries, but is not as effective against anatomical reentries. Generally, if ATP is not effective, a defibrillation shock of large amplitude is applied directly to cardiac muscle.
Such high voltage, high energy shocks may be delivered by a standard external defibrillator with the patient sedated during delivery of a defibrillation shock. However, in order to provide an external shock that can effectively terminate arrhythmias with electrodes placed externally on the body, such systems must provide higher energy shocks than would be required by implantable devices. In addition, externally applied shocks necessarily recruit more of the skeletal musculature resulting in potentially more pain and discomfort to the patient.
Another method of treatment for patients experiencing ventricular tachycardia (VT) or ventricular fibrillation (VF) is the implantable cardioverter defibrillator (“ICD”). However, the energy level needed for successful cardioversion can also exceed the pain threshold. Endocardial cardioversion shock energies greater than 0.1 J are perceived to be uncomfortable (Ladwig, K. H., Marten-Mittag, B., Lehmann, G., Gundel, H., Simon, H., Alt, E., Absence of an Impact of Emotional Distress on the Perception of Intracardiac Shock Discharges, International Journal of Behavioral Medicine, 2003, 10(1): 56-65), and patients can fail to distinguish energy levels higher than this and find them equally painful. The pain threshold depends on many factors, including autonomic tone, presence of drugs, location of electrodes and shock waveforms. Moreover, pain thresholds can be different from patient to patient. Further, as compared to external defibrillators, ICD's present other challenges, including a limited energy source.
Many systems have sought to lower the energy level required for effective atrial fibrillation. A number of systems, such as, for example, U.S. Pat. No. 5,282,836 to Kreyenhagen et al., U.S. Pat. No. 5,797,967 to KenKnight, U.S. Pat. Nos. 6,081,746, 6,085,116 and 6,292,691 to Pendekanti et al., and U.S. Pat. Nos. 6,556,862 and 6,587,720 to Hsu et al. disclose application of atrial pacing pulses in order to lower the energy level necessary for atrial defibrillation shocks. The energy delivered by pacing pulses is relatively nominal in comparison to defibrillation shocks. U.S. Pat. No. 5,620,468 to Mongeon et al. discloses applying cycles of low energy pulse bursts to the atrium to terminate atrial arrhythmias. U.S. Pat. No. 5,840,079 to Warman et al. discloses applying low-rate ventricular pacing before delivering atrial defibrillation pulses. U.S. Pat. No. 5,813,999 to Ayers et al. discloses the use of biphasic shocks for atrial defibrillation. U.S. Pat. Nos. 6,233,483 and 6,763,266 to Kroll discloses the use of multi-step defibrillation waveform, while U.S. Pat. No. 6,327,500 to Cooper et al. discloses delivering two reduced-energy, sequential defibrillation pulses instead of one larger energy defibrillation pulse.
However, reduced-energy AF treatments do not necessarily translate well to VT or VF treatments in part due to the physiological differences in the causes of AF vs. VF, but also in part due to the criticality of VT and VF.
Consequently, there remains a need for improved VT and VF treatment methods and devices enabling successful electrical treatment without exceeding the pain threshold of a patient.